1. Technical Field
The invention relates to optical devices and in particular to so-called opto-mechanical devices, in which the switching of, or other operation upon, optical signals is achieved by displacing an optical element, such as an optical waveguide, a mirror, a prism, and so on. Embodiments of the invention are especially applicable to optical telecommunications systems.
2. Background Art
Within optical telecommunications systems, the applications for optical switches are generally analogous to those for electrical switches in conventional telecommunications systems. Thus, the system might use optical switches of several different kinds with various performance characteristics according to the particular application. For example, the switches might range from simple 1.times.2 port switches to complex multi-port switches and switching speeds might vary over a wide range. Although many alternative switching technologies have been demonstrated, so-called "opto-mechanical" switches are preferred for most, if not all, practical applications. In such opto-mechanical switches, the switching function is achieved by physically displacing an optical element, which might be a mirror, prism or even an optical fiber carrying the signal. For example, a known 1.times.2 switch operated electromagnetically comprises a tube with a pair of permanent magnets on either side of the middle of the tube. A cylindrical solenoid coil is provided inside the tube. Two fibers protrude into the tube from one end and are located in opposed V-grooves in respective half-cylindrical ferrules. A single, movable optical fiber protrudes into the tube from the other end to such an extent that its end protrudes into the opposed V-grooves. A magnetic alloy sleeve is attached to the middle of the movable fiber. Operation of the electromagnet displaces the magnetic alloy sleeve towards one or other of the permanent magnets, causing the end of the movable fiber to align with one or other of the stationary fiber ends. A disadvantage of such a switch is that curvature of the flexed fiber may lead to problems in aligning the end of the movable fiber with the fixed fibers. Greater precision might be achieved by replacing the magnetic sleeve with a sliding mounting, but the problem of moving the mounting precisely then arises.
For more complex switching functions, it is known to make a 4.times.4 matrix switch from six 2.times.2 movable optical elements disposed in an optical path between an input lens array and an output lens array, each array comprising four rod lenses coupled to a corresponding number of optical fibers. The rod lenses collimate and focus light beams passing between the input and output fibers. Each movable optical element consists of a rhombic glass block which can be slid to and fro transversely relative to the arrays by means of electromagnets. In one position, the "straight-through" position, the glass block causes two adjacent light beams to pass through it in parallel. In the other position, the "crossover" position, the glass block causes the two beams to cross over. Hence, sliding the glass blocks to and fro changes the coupling between the input and output fibers.
In many situations, such opto-mechanical switches will perform satisfactorily for long periods since, unlike electro-mechanical switches, they do not have electrical contacts susceptible to contact resistance problems. However, in other situations, they may be unsatisfactory. Usually, the input and output ports of the switch will be single mode optical fibers with core diameters of about 8 micrometers, so the displacement of the movable element must be extremely precise and extremely stable mechanically. Where pivoting or sliding mechanisms are used to move the optical element, mechanical stability may be achieved by using relatively large moving parts. Unfortunately, this requires the actuator, i.e. solenoid, relay armature, and so on, to be relatively large. Such increased size may lead to difficulties and increased costs, both during manufacture and when the switch is installed.
Them are some applications where known optical switches are unsatisfactory for other reasons. For example, in a telecommunications system which employs a ring system architecture, if a local station fails, the main traffic can be re-routed by a routing switch which operates and by-passes the failed station. The routing switch may never need to operate throughout the lifetime of the system. If and when it does, however, it must be totally dependable, especially if it is installed in a submarine system. Where sliding or pivoting parts are used to move the optical element, they may be affected by corrosion, rusting, oxidation, and so on, causing binding of the drive mechanism, especially in such submarine applications or other adverse environmental situations.
Where a switch is subject to repeated operation, its reliability and dependability over its expected lifetime can be assessed by accelerated usage to simulate such repeated operation. However, this approach is unsuitable for ensuring that a switch will operate after such a long period of inactivity. It is desirable, therefore, for the switch design to be inherently dependable.
Thus an object of the present invention is to mitigate the problems of known opto-mechanical switches and to provide an improved opto-mechanical switch which is dependable where operation is infrequent.